Devices for determining drowsiness (degree of awakening) of a driver of a vehicle have been developed in order for the driver to drive safely. An example of the drowsiness determining devices extracts a characteristic amount changing according to drowsiness from heartbeats and blinks, for example, and determines very strong drowsiness (a dozing state or a state immediately before the dozing state). When such very strong drowsiness is determined, its drowsiness level is such that an influence (e.g., faltering) appears in a driving operation, whereby the determination timing is late. Hence, the device disclosed in Patent Literature 1 determines weak drowsiness according to a heartbeat fluctuation low frequency component which is correlated with a sympathetic activity, and provides a stimulus when the weak drowsiness is determined.    Patent Literature 1: Japanese Patent Application No. 2007-140975    Patent Literature 2: Japanese Patent Application Laid-Open No. 2007-6970    Patent Literature 3: Japanese Patent Application Laid-Open No. 6-270711